Field emission displays include an anode and a cathode structure. The cathode is configured into a matrix of rows and columns, such that a given pixel can be individually addressed. Addressing is accomplished by placing a positive voltage on one row at a time. During the row activation time, data is sent in parallel to each pixel in the selected row by way of a negative voltage applied to the column connections, while the anode is held at a high positive voltage. The voltage differential between the addressed cathode pixels and the anode accelerates the emitted electrons toward the anode.
Field effect devices typically comprise a metal cathode on a substrate, with carbon nanotubes grown on the cathode. A metal catalyst may be positioned between the cathode and the carbon nanotubes for facilitating carbon nanotube growth. A gate electrode is positioned between an anode and the tops of the carbon nanotubes for controlling electron emission from the carbon nanotubes. Electrons flow from the metal cathode through the metal catalyst if present, and out the carbon nanotubes to the anode spaced therefrom.
Color field emission display devices typically include a cathodoluminescent material underlying an electrically conductive anode. The anode resides on an optically transparent frontplate and is positioned in parallel relationship to an electrically conductive cathode. The cathode is typically attached to a glass backplate and a two dimensional array of field emission sites is disposed on the cathode. The anode is divided into a plurality of pixels and each pixel is divided into three subpixels. Each subpixel is formed by a phosphor corresponding to a different one of the three primary colors, for example, red, green, and blue. Correspondingly, the electron emission sites on the cathode are grouped into pixels and subpixels, where each emitter subpixel is aligned with a red, green, or blue subpixel on the anode. By individually activating each subpixel, the resulting color can be varied anywhere within the color gamut triangle. The color gamut triangle is a standardized triangular-shaped chart used in the color display industry. The color gamut triangle is defined by each individual phosphor's color coordinates, and shows the color obtained by activating each primary color to a given output intensity.
However, vacuum field emission devices are commonly plagued with electrons being emitted (a leakage current) from various types of unintended emission sites. These spurious emission sites are often formed as an unintended consequence of the fabrication process. Unintended emitters can result from anomalously sharp edges of metal electrodes; conductive particles in high field regions, patterning defects, lifting metal, emitters (such as nanotubes) deposited in the wrong place, etc. In addition, many types of field emission cathode structures have a gate electrode stack. This feature typically incorporates a metal gate electrode deposited on top of an insulator, which is then deposited on or very near a cathode electrode. The edges of these features are typically exposed at a sidewall feature. In some cases the wall in vertical, in some cases the ‘wall’ is a gentle slope, and in some cases, the wall is a concave feature. Regardless of the exact structure of the wall, the ‘wall’ feature is a typical location for unintended emission because it is a high electric field region.
An example of a type of defect that causes unintended emission is a sharp point on the sidewall of the gate electrode metal. This defect can occur at the edge of a gate electrode stack, but it can also occur at any edge of the gate metal. This defect is typically caused by a wet etch in the manufacturing process, but could also be caused by lithography, stamping, screen printing, or any other process providing gate anomalies. In the case where the anode field alone is sufficient to initiate electron emission, this undesired emission site is commonly referred to as an anode leader. The intensity of electron emission increases with the applied anode voltage. Furthermore, when field emission devices are in their ‘off’ state, the gate electrode potential is driven lower than the cathode electrode potential, creating a reverse bias condition. In this case, the cathode electrode itself provides the field which pulls electrons off the gate metal asperity. This emission site is often called a reverse bias leader. Both cases lead to image defects wherein the sub-pixels are always illuminated, resulting in loss of contrast and brightness, and the inability to operate the device at optimal conditions.
Another type of unintended emission results from defects at the edge of the gate electrode stack. Conductive particles can be defects at the base of the gate electrode stack. They might result from particles present in the process line, patterning defects, re-deposited material during wet processing, or emitter features (such as nanotubes) erroneously deposited in the wrong place. The base of the gate electrode stack forms a junction between a conductor, an insulator, and vacuum which is commonly termed a triple point. This junction creates an enhanced electric field at the conductive defect, and under the influence of the gate potential and/or the anode potential, the conductive defect can emit electrons. These electrons typically cascade up the sidewall, producing an unwanted leakage current between the anode and the cathode, and often produce emitted electrons at the anode. These defects typically are not ballasted by series resistance in the field emission structure so they contribute to excessive (and non-uniform) light at the sub-pixel. They also become hot and produce a run-away current condition that ends in the explosion of the defect, and sometimes a device shorting defect.
Another defect, residual conductive material on the gate electrode stack sidewall, can also produce leakage current between the gate and the cathode electrodes. The residual conductive material allows some electrons to pass between the gate and cathode electrodes along the insulator surface (surface hopping of emitted electrons). With higher bias between the electrodes, more current flows. Sufficient current flow causes the region to heat up and the current to increase in a positive feedback condition that often ends in an explosion of the sidewall region. This may produce a device shorting defect.
Accordingly, it is desirable to provide a method of preventing electron emission from various defects at the sidewall of a gate electrode and the edges of the gate electrode stack of a field emission device. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.